The use of membranes for separating various components of gas mixtures is generally known. Membranes have been used in order to separate, to remove, to purify or partially to recover many gases.
The separation of hydrogen from hydrogen-comprising hydrocarbon mixtures or from carbon monoxide and/or carbon dioxide is of particular interest at present. For example, such a separation is required in the preparation of propene from propane (dehydrogenation), of synthesis gas (adjustment of the H2/CO ratio) or of hydrogen via the water gas shift reaction (reaction of CO with water to form CO2 and H2). In the prior art, predominantly polymer membranes are used for separating hydrogen and/or water. For example, WO 08/46880 describes the use of polyimide membranes. The polymer membranes can generally be used at a temperature of from 20 to 80° C., in some cases up to 120° C. However, since the hydrogen-comprising gas mixtures are obtained at high temperatures of from 200 to 600° C. in most industrial processes, they have to be cooled before contact with the polymer membrane. This cooling represents a complicated and costly process step.
There is therefore a great need for membranes which can separate hydrogen from hydrogen-comprising mixtures in gas or vapor form at elevated temperatures.
Zeolites which are low in aluminum or comprise essentially only SiO2 are generally known as thermally stable materials to those skilled in the art. The advantage of zeolites is that they have a crystalline structure and a defined pore size. Owing to their specific topology, zeolites can accordingly be used for separation applications by means of molecular sieving, i.e. molecules having a kinetic diameter which is smaller than or similar to the diameter of the largest zeolite pores can penetrate into the framework structure and are adsorbed or, as a result of a driving force, transported through the framework structure, while larger molecules are shut out or held back. The gas or vapor molecules present in the mixture which can be separated by the principle of molecular sieving are therefore fixed for a given zeolite framework type which defines the topology of the framework and pore structure.
The prior art describes, for example, zeolite membranes of the pure silica MFI type (MFI has 10-membered rings; e.g. Kalipcilar et al.: effect of seeding on the properties of MFI type zeolite membranes; Desalination 200 (2006) 66-67) or DDR type (DDR has 8-membered rings, NGK Insulators: porous structure with seed crystal-containing layer for manufacturing zeolite membrane and method for manufacturing zeolite membrane, EP 1894613); these zeolites have pore diameters of >0.35 nm and thus do not have a pore size in the region of the gas diameter of hydrogen.
The zeolite membranes described in the prior art are usually produced by the “secondary growth” method (see, for example, van der Donk et al. in Microporous and Mesoporous Materials 115 (2008) 3-10). Here, seed crystals are applied to a porous inorganic support which may be metallic or ceramic in nature by filtering on or by attachment using polymeric quaternary ammonium compounds and subsequently fixed by sintering. After sintering, the rear side of the future zeolite membrane is covered so as to make it impervious to liquid and the support is introduced into a synthesis solution comprising water, organic structure formers (templates) such as tetrapropylammonium hydroxide, 1-aminoadamantane or quinuclidine, an SiO2 source such as TEOS (tetraethyl orthosilicate) or silica sols (dissolved polysilicic acids or dispersions of amorphous silicon dioxide) and, optionally, inorganic auxiliaries such as bases, salts, etc. The hydrothermal crystallization of a dense zeolite layer onto the support is then carried out in the synthesis solution at temperatures of from 80 to 180° C. A good overview on the subject of zeolite membranes and the synthesis of zeolite membranes is given by, for example, Caro et al. in Microporous and Mesoporous Materials 115 (2008) 215-233 (zeolite membranes—recent developments and progress).
Clathrasils, on the other hand, whose framework comprises essentially SiO2 and no further metals such as Al are, owing to the framework structure which has a 6-membered ring pore system having a (temperature-dependent) pore diameter of about 0.25-0.27 nm, are mainly suitable in processes of gas separation, vapor permeation or pervaporation for separating small molecules such as hydrogen (kinetic gas diameter of 0.289 nm), helium (kinetic gas diameter of 0.255 nm) or water (kinetic gas diameter of 0.290 nm) by the principle of molecular sieving from larger gas molecules such as CO, CO2, CH4 and higher aliphatic or aromatic hydrocarbons (kinetic gas diameter of >0.3 nm) on the basis of the size difference. In this context, the clathrasils of the framework type SOD, AST and SOT are of particular interest since these have the highest density of 6-membered rings and a 3-dimensional pore system.
However, the prior art gives no information as to how a clathrasil membrane is to be produced. For example, Münzer et al, (Microporous and Mesoporous Materials 110 (2008) 3-10) discloses that no success was achieved in producing suitable seed crystals from a pure silica SOD zeolite by means of various organic compounds as structure-directing reagents (templates) for the secondary growth method. It was only possible to produce a small amount of nanocrystals which was obtained together with a large number of large particles and unreacted amorphous material. Furthermore, the nanocrystals could be isolated only with great difficulty. As a result, the production of a clathrasil membrane has hitherto foundered on the provision of clathrasil seed crystals.
Likewise, van der Dank et al. (Microporous and Mesoporous Materials 115 (2008) 3-10) have not succeeded in preparing selectively separating D1H membranes (D1H is the pure silica variant of the 6-membered ring structure type DOH). Although virtually template-free crystals could be produced by milling larger crystals and heating in air at elevated pressure and a temperature of 900° C., this was with some loss of crystallinity. However, the layers produced by the secondary growth method on an aluminum oxide support did not display a closed layer after template removal for 5 hours at 700° C. but instead displayed intercrystalline holes and partial detachment and are thus unsuitable as gas separation membranes. Whether the partially intact regions of the D1H layer were template-free is not disclosed.
The crystal geometry of the SOD clathrasil known in the prior art is cubic; the crystals therefore predominantly have a cubic, octahedral or rhombododecahedral morphology. Münzer et al. discloses, for example, the tetramethylammonium AlSi SOD in cube form (Microporous and Mesoporous Materials 110 (2008) 3-10, FIG. 3). AST is likewise cubic and displays the octahedron as crystal morphology. On the other hand, clathrasil SGT is tetragonal and displays capped bipyramids as crystal morphology.
The preparation of template-free clathrasil is itself very complicated since removal of the template occurs only at temperatures of from 800 to 1000° C. and has to be carried out for a number of hours to weeks. It is also difficult to keep the zeolite structures intact at the high temperatures over the long reaction time.
In Angew. Chem. 1996, 108, No. 23/24, pages 3041 to 3044, Oberhagemann et al. disclose that sheet silicate structures can serve as zeolite precursors. For example, FER was obtained by simple heating of PREFER and MWW was obtained from MCM-22. Oberhagemann et al. assume that RUB-15 goes over into the zeolite sodalite on suitable temperature treatment. Unfortunately, this assumption that a pure temperature treatment of RUB-15 leads to sodalite has not been confirmed, as example 5 shows.
Topotactic condensations have also been described, for example, for the conversion of RUB-18 into RUB-24 (Marler et al.: The structure of the new pure silica zeolite RUB-24, obtained by topotactic condensation of the intercalated layer silicate RUB-18, Microporous and Mesoporous Materials 83 (2005) 201-211) or of octosilicate into RWR (Oumi et al.: Convenient conversion of crystalline layered silicate octosilicate into RWR-type zeolite by acetic acid intercalation, New Journal of Chemistry 31(2007), pp. 593-597). However, the zeolites obtained have rings having more than 6 linked SiO4 tetrahedra and are thus not suitable for the separation task.
Compared to the use of clathrasils in powder form as absorbent, a clathrasil membrane offers the opportunity of separating molecules by means of a continuous process, which can prove to be of particular interest from both a technological point of view and an economic point of view.